My stance on “Repairing Permissions” is not complicated. If you have a permissions-related problem (or suspect one) it might solve it. It is not something you should worry about if you’re not having any problems. There is no good reason to run it periodically, nor any reason to run it before or after software installations or upgrades. If it were a good idea, Apple would configure Mac OS X to do it for you. It’s like taking antibiotics — a good idea if you have an infection, but a bad idea to take every day “just for good measure”. (My thanks to “Floggy”, whoever you are, for making the same point in the comments on TUAW.)